Welcome!
Every kid is different some kids are total squirrels when they first start out and some hop on and end up being naturals. Your best bet would probably be to get him one of the bikes Mike listed above, they are very tame. The klx110/drz110, xr70, crf70, pw80 will all do just fine for track use while your son is still in the beginner stages. If your kid already has a good grasp on driving/riding motorized vehicles I say go right for a 65. In addition to the Ktm 65sx there's also the Kawsaki KX65 to consider. I made a smooth transition to the kx60 after only riding a pw50 and here I am still alive today!
If you could explain further on what you'd like to know about gear we might be able to help you out!